1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus for recording information in an information recording medium having a single recording layer or a plurality of recording layers, and for reproducing the information recorded in the information recording medium.
2. Description of the Related Art
A digital versatile disc (DVD) is a well-known information recording medium (optical disc) which can store a large quantity of data. For example, it can store digital information at a recording density about 6 times greater than that of a compact disc (CD). In recent years, as the amount of information to be recorded in an information recording medium has increased, there has been demand for an information recording medium having a larger recording capacity. In order to increase the recording capacity of an information recording medium, it is necessary to increase the information recording density by reducing the size of a spot of light which is emitted on an information recording medium when recording information in, or reproducing information from, the information recording medium. For example, the size of the light spot can be reduced by using laser light having a shorter wavelength and increasing the numerical aperture (NA) of an objective lens of an optical pickup. In an existing DVD recording/reproducing apparatus, a light source having a wavelength of 660 nm and an objective lens having a NA of 0.6 are generally used. For example, a blue light laser having a wavelength of 405 nm and an objective lens having a NA of 0.85 can achieve a recording density 5 times greater than the currently-achieved DVD recording density.
In addition to development of a recording/reproducing apparatus which uses a blue light laser having a shorter wavelength, an information recording medium having a plurality of recording layers, which may achieve a 2 times greater recording density, has been developed. If an optical disc having a two recording layers is realized in addition to a reduction of laser light wavelength and an increase in the NA of objective lens, a recording density about 10 times greater than that of an existing DVD having a single recording layer can be achieved.
However, in a high-density optical disc apparatus which uses a blue light laser as a light source, a margin of the power of light generated by a blue light laser for reproduction is very small, and a light source for such a blue light laser has a problem of quantum noise. An optical head which addresses the problem of quantum noise is disclosed in Japanese Laid-Open Publication No. 2000-195086. The optical head disclosed in this document can perform high quality reproduction while the light power on a surface of an optical disc is kept low, and deterioration of an optical disc and erroneous data deletion are prevented, and the amount of quantum noise in a semiconductor laser is kept small.
FIG. 6 shows a structure of a prior art optical head 600.
The optical head 600 includes a light source 161, an intensity filter 162, a beam splitter 163, a collimator lens 164, a mirror 165, an objective lens 166, a multi-lens 168, and a photodiode 169.
The light source 161 is a GaN-based semiconductor laser which emits blue light. The light source 161 also emits coherent light used for recording/reproduction in a recording layer of an optical disc 167.
The intensity filter 162 is an optical element including an absorber film which absorbs light. In the optical system illustrated in FIG. 6, the intensity filter 162 can be mechanically inserted in or removed from the optical path of the optical system.
The beam splitter 163 is an optical element which splits a light beam emitted by the light source 161. The collimator lens 164 is a lens for converting the light beam emitted by the light source 161 into parallel light. The mirror 165 is an optical element which reflects an incoming light beam and directs the reflected light beam to the optical disc 167. An objective lens 166 is a lens for condensing the light beam onto a recording layer of the optical disc 167. The multi-lens 168 is a lens for condensing the light beam onto the photodiode 169. The photodiode 169 receives the light beam reflected by a recording layer of the optical disc 167 and converts it into an electric signal.
Next, an operation of the optical head 600 is described.
When information is recorded on the optical disc 167, the intensity filter 162 is out of the optical path of the optical system. The light source 161 emits a light beam which is modulated according to information to be recorded. The light beam is reflected by the beam splitter 163 and converted into parallel light by the collimator lens 164. Then, the light beam is reflected by the mirror 165 and transmitted through the objective lens 166 so as to be condensed on the optical disc 167 (i.e., a light spot is formed on the optical disc 167). The state of the recording layer in the light spot portion is changed (for example, the crystalline state of the light spot portion is changed) according to the information to be recorded, whereby information is recorded on the optical disc 167 as a change in the state of the recording layer.
In the case of reproducing information recorded in the optical disc 167, the intensity filter 162 is placed in the optical path. The light source 161 emits an unmodulated light beam. The light beam emitted by the light source 161 is transmitted through the intensity filter 162 so that the amount of light (power) of the light beam is attenuated. The attenuated light beam is reflected by the beam splitter 163 and converted into parallel light by the collimator lens 164. Then, the light beam is reflected by the mirror 165 and transmitted through the objective lens 166 so as to be condensed on the optical disc 167. The light beam is reflected by the recording layer of the optical disc 167 at a reflectance according to the state of the recording layer. The light beam reflected by the recording layer is transmitted through the objective lens 166 and reflected by the mirror 165 again. Thereafter, the light beam is transmitted through the collimator lens 164 and the multi-lens 168 so as to be condensed onto the photodiode 169. The photodiode 169 extracts: an information signal which indicates information recorded on the optical disc 167; a focusing error signal which indicates a focusing state of the light beam on the optical disc 167 using an astigmatism method; and a tracking error signal which indicates the irradiation position of the light beam. A focus control section (not shown) controls the position of the objective lens 166 along the optical axis direction based on the focus error signal, thereby condensing the light beam onto the optical disc 167 in a focused state. A tracking control section (not shown) controls the position of the objective lens 166 in a direction perpendicular to the optical axis direction based on the tracking error signal, thereby condensing the light beam onto a desired track of the optical disc 167. The photodiode 169 also reproduces the information signal. By such an operation, information recorded in the optical disc 167 is reproduced.
The prior art optical head 600 can set the light power of the light source 161 to a level such that the quantum noise is kept sufficiently small during a reproduction operation, and can reproduce information while the light power at a surface of an optical disc is kept low such that deterioration of the optical disc and erroneous deletion of data can be prevented. On the other hand, during a recording operation, the optical head 600 can perform recording of data without reducing the light power of the light source 161.
However, in the optical head 600 having the above structure, it is necessary to mechanically insert the intensity filter 162 in or remove the intensity filter 162 from the optical path when the operation mode is switched between recording and reproduction operations. When a recording operation is performed immediately after reproduction is performed at an address of the optical disc 167, the speed of moving the intensity filter 162 is a problem to be considered. For example, in a next-generation high-density optical disc which has a higher recording density than that of a DVD, it is necessary to perform switching of the operation mode within about 100 nanoseconds. However, such a quick switching is impossible with the optical head 600 because the intensity filter 162 is mechanically inserted in or removed from the optical path in the optical head 600. Even when a liquid crystal device is used as the intensity filter 162, it is difficult to achieve such a quick switching of the operation mode.
Furthermore, in the case of recording/reproducing information in/from a information recording medium having one type of recording sensitivity (i.e., a recording medium having one recording layer), a semiconductor laser of a certain type can emit a light beam with reduced quantum noise onto an information recording medium without reducing the light power of the light beam by an intensity filter during a reproduction operation. However, in the case where information recording media having different recording sensitivities, for example, an information recording medium having a single recording layer and an information recording medium having two recording layers are recorded or reproduced using the same optical head, it is difficult to reduce the quantum noise of the light beam. In general, a light beam used for recording/reproducing information in an information recording medium having two recording layers requires light power about 2 times greater than that required for a light beam used for recording/reproducing information in an information recording medium having a single recording layer. It is difficult to obtain a light beam with reduced quantum noise for reproducing information from an information recording medium having a single recording layer while obtaining the light power of the light beam required for recording the information recording medium having two recording layers.